Band pass network



||II I I IIIII INV H/vEg-TOR Patented Oct. 22, 1935 BAND PASS NETWORKJohn S. Low, Berkeley, Calif.; Herbert W. Low, administrator of saidJohn S. Low, deceased, assignor to American Telephone & TelegraphCompany, Incorporated, New York, N. Y., a corporation of New YorkApplication August 19, 1929, Serial No. 386,786 Y 15 Claims.

My invention relates to selective networks, and particularly to networkssuch as are used in radio receiving apparatus. Itsbroad purpose is toprovide a network which may be tuned over a range of frequencies, andwhich will pass a limited and substantially constant width frequencyband, on each side of the frequency to which it is tuned, over itsentire tuning range.

An object of my invention is to provide a means of accurately adjustingthe width of the pass band of a tuned network.

Another object of my invention is to provide a tunable band pass networkwhich is not dependent upon magnetic coupling, and in whichthe'inductance coils may be completely shielded.

Still another object of my invention is to provide a selfcontainedtuning and coupling unit for use with such a band pass network.

A further object of my invention is to provide a band pass network whichmay be used either as a purely selective unit, or which may be used as acoupling unit between the stages of an amplier.

My invention possesses other objects and valuable features, some ofwhich will be set forth in the following description of my inventionwhich is illustrated in the drawing forming part of the specification.It is to be understood that I do not limit myself to the showing made bythe said description and drawing, as I may adopt varying forms of myinvention within the scope. of the claims.

Referring to the drawing:

Figure 1 is a circuit diagram of the network of my invention as used forcoupling two tubes of a radio frequency amplifier. v

Figure 2 is a perspective view of a condenser adapted for use as acombined tuning and coupling device in the network of my invention.

In broadly descriptive terms the network of my invention comprises apair of tuned circuits having a common reactor, and a bridging'V reactorof similar phase characteristics to the common reactor connecting thecircuits at a point remote I from the common reactor. In its preferredform capacity reactors .are used as the common and bridging reactors,and the bridging reactor may be made as a part of the tuning'condenserunit used with the circuit. t K

In more detailed terms, Fig. 1 isa diagram of the network used as anintertube'coupler in a radio frequency amplifier, In order .to simplifythe drawing but two tubes 6 and 1 of such an amplifier are shown, theinput circuit to the tube 6 being represented by animpedance 8, and

the output circuit by an impedance 9. These circuits may be networkssimilar to that to be dee scribed in detail, or they may be conventionalor special circuits ofother types.

The vacuum tubes shown are conventional triodes having filaments I I,control electrodes or grids I2, and anodes or plates I3. The triodesshown are the familiar battery type tubes, whose filaments are suppliedby an A battery I4 and whose plate circuits are supplied by a B batteryI6, but it is obvious that alternating current tubes may be substitutedor that tetrodes maybe used in place of the triodes, without affectingthe performance of the band pass network which connects them. 1

Connected to the plate of the tube 6 is a tuned resonant circuitcomprisingan inductor I'I and a variable condenser I8. A similar tunedcircuit comprising an inductance I9 and a variable condenser 2I isconnected to the grid of the tube 1. One of the sets of elements of thecondensers I8 and 2| are connected together, and to the junction ofthese condensers is connected a common fixed condenser 22, the otherside of which connects to the low potential end of the two inductors I'Iand I9.y rIhe connection between the condenser 22 and the inductor I9 isdirect; that between' the inductor I1 and the condenser 22 is thru ablocking condenser 23 which is of a suiciently large value to have noappreciable effect on the tuning of the resonant circuit, that is, it isof one to two orders of magnitude larger than the condenser 22.

A lead 24 connects from the battery I6 to the low potential end of theinductor I1 and thence to the plate of the tube I5. The low potentialend of the inductor I9 connects tothe filament circuit of the tube,preferably thru a biasing hat-` tery 26. The inductors I1 and I9 arepreferably completely enclosed in shields 21.

A bridging condenser 28 lconnects the high potential end of the coils I1and I9. This condenser is of very small capacity as comparedA to thecondensers I8.'r The condenser 22 is of very large capacity 'as comparedto the condenser I8. The method of selecting these capacities will nextbe considered..

The coupling network consists of two resonant circuits which are veryloosely coupled. It is well known that the resonancev curve of such apair of coupled' circuits differs materially from that of either circuitconsidered by itself. As#A suming that the two circuits aresubstantially identical and tuned to the same frequency, there is acertain value of coupling, known as critical A response curve, whilemore than critical coupling 'zoVV equal to L,

also lessens the response but results in a concave top curve, that is,in a curve having two humps or maxima. u i f If f is the frequency towhich the circuits considered individually are in/resonance, Vthefrequencies of maximum response of the circuit Will be very nearly andfen/1 k,

wherelcis the coefficient ,of couplingi Y Experience has shown that forradio telephone reproduction it is desirable to have the circuit respondsubstantiallyuniformally over a band Vof about 10 kilocycles', i. e. aband 5,000 cycles on either side of the frequency f of the carrierwave.' Experience has further shown that if f and f are about 5,000cycles apart or half the Width ofV the band, this result is very closelyapproximated. Owing to the variation of resistance in radio frequencycircuits with frequency, and the corresponding change in shape of theresonance curve, it is desirable that j and ff be slightly fartherraparton the lower fre- Yquencies and slightly closer on the higher, but

the rule given is suiiiciently accurate for a rst approximation. Y' AWhere 7c is small the difference f-f is very nearly f k. Itis thereforeevident that if 1c is a constant, the band passed Vby such a networkvaries almost directly as f. Consideredin terms of the present radiobroadcasting frequency spectrum, from 550 kilocycles to 1500 kilocycles,

the width of the pass band will be approximately three times as great atthe high frequency end of the band asjat the low frequency end. In orderto pass a band of constant width, lc must be made to vary 'substantiallyinversely as the frequency. Y

In the network here described lc comprises the sum of two components,the first being due to the coupling introduced byV the condenser 22which may be termed k1, while the other is due to the condenser 28 andmaybe termed k2. Calling the capacity of condenser 2,2 C1 the capacityof condenser 28 Cz, and the capacity of condensers I8 and. 2|C,

lc""C-l- 0,

and

As "C is the tuning condenser, its capacity varies with the frequency towhich the network is tuned, and if the inductances l1 and I9 are eachSubstituting in the equations for k1 and k2,

k 1 k C2 21rf 2L 1 1+(21rf)2LC1, 27`1+C2 2m2L Since both k1 and Icz arevvery small, the ex- 5 pressions Y and k2=(21rf)2LC2 are suiiicientlyaccurate for the purposes of computation. lo

If, as is desirable in networks of this char--V acter, the ratio ofinductance to resistance in the coils is high, adesirable pass band isob- `tained' by making f-j=6,000 cycles at f=550 kilocycles, andf-f=4,000 cycles at f=1,500 15 kilocycles. Hence, takingf-f=lcf=(lc1+k2)f, suitable values for 1c are .0109 at 550 kc., and

, 1c=.0027 at 1500 kc. Y

We may now put 20 k1+kz=lc at the two extremefrequencies in the band,and substituting in the above equations for k1 and k2, and taking 200microhenries as a suitable value for L, we obtain the simultaneousequations 25 Itwill be realized that While the former capacity isrelatively large as radio capacitances go, the capacitance Cz is sosmall as to be Ywith difficulty obtainable. The method of securing anaccurate capacitance of this. value which I prefer, is to 0.

the dotted lines 39, surrounds the individual tuned 5o circuits. Theshield 4| between the two stator sections, forms a part of Vthe generalshielding, and this shield is cut-away as shown atv 42' to provide acapacitance of the proper magnitude between the stator members. The arealof the 5'5"- opening inthe shield depends,V of course, both upon themagnitude necessaryrfor the coupling capacity C2 and upon thel spacingof the stators, the. latter being determined by mechanical con#siderations,VV Y

I am 4aware that it has been proposed to construct `constant width tunedband pass networks by a combination of capacity and inductive coupling.This system, however, inevitably fintro-V duces accidental capacitycouplings `between the 05' inductors which practically vitiate itsusefulness. The method of Ymy invention, however, enables Y the designerto reduce theinductive couplings to substantially* nil, leaving only.the precalculate capacity couplings to aifect -thecircuits p `Thefigures given in the above example Aare to Vbe considered asillustrative only,VV and have not been determined with greater than.slide rule accuracy. Second'r order .innitesimals have,

moreover, been persistently neglectedV both in the 'YB The stator 5.1

derivation of the formulas and in the computations. The results,however, are-suiiiciently accurate for all ordinaryV purposes, and exactresults may be obtained by methods which are published in many texts andare familiar to those skilled in the art. i

I claim:

1. A band-pass network comprising a pair of tuned circuits having acommon capacitor therein and a bridging capacitor between saidcircuits,said capacitors being proportioned to provide a substantially constantwidth of pass bandas said circuits are tuned syntonously over a range offrequencies.

2. A band pass network comprising a pair of similar tuned circuitshaving a common capacitor therein, and a bridging capacitor between saidcircuits, saidcapacitors being proportioned to provide a substantiallyconstant width of pass band with variable tuning of said circuits.

3. A band pass network comprising a pair of tuned circuits eachcomprising a fixed inductor and a variable capacitor, a capacitorcom-mon to said circuits and having each of said inductors andcapacitors connected thereto, and a bridging capacitor connecting thejunctions of said inductors and variable capacitors remote from theirconnections to said fixed capacitor.

4. A tuned band pass network comprising a pair of resonant circuitshaving fixed inductors with substantially zero mutual inductance andvariable capacitors, a capacitor common to said circuits, and a bridgingcapacitor connecting said circuits at a point remote from said commoncapacitor.

5. A tuned band pass network comprising a pair of resonant circuitshaving fixed inductors with substantially zero mutual inducance andvariable capacitors having a mutual capacitance `between the elementsthereof, and a capacitance common to said circuits.

6. A band selective transmission network comprising a par of syntonoustuned circuits, each comprising an inductance and a variable tuningcapacity, means for varying the tuning of said circuits simultaneously,coupling means comprising a capacity common to both of said circuits andadditional coupling capacity connecting said tuned. circuits serially,said common capacity being large compared with the tuning capacities ofsaid tuned circuits and said series capacity being small relativelythereto.

'7. A band-pass network comprising a pair of tuned circuits eachcomprising a iixed inductor and a variable capacitor, means for varyingthe tuning of said circuits simultaneously, a capacitor common to saidcircuits and having one terminal of each of said variable capacitorsconnected to one terminal thereof, additional coupling capacity betweenthe other terminals of said variable capacitors, said additionalcapacity being constituted by direct capacitance between said variablecapacitors and partial shielding capacitance between said variablecapacitors and `5 being proportioned to cooperate with said commoncapacity to maintain the pass-band substantially constant in widththroughout the tuning range of the network. f

9. The combination with two serially connected inductances havingsubstantially yzero electromagnetic coupling, of a capacitive couplingbetween the non-common ends of said inductances,

a condenser having one terminal connected to the junction of saidVinductances, a pair of'tuning 15 condensers having a common terminalconnected to the other terminal of said rst condenser, the oppositeterminals of said tuning condensers being connected to the respectivenon-common terminals of said inductances, and mechanical meansconnecting the adjustable elements of said tuning condensers to insuretheir simultaneous adjustment, the tuning condensers and inductanceshaving such values that the two tuned circuits remain continuously inresonance throughout the range of adjustment of said tuning condensers.

10. A high frequency network comprising a pair of resonant oscillationcircuits having fixed inductors with substantially zero mutualinductance between them, variable tuning condensers connected to saidinductors having a mutual capacif tance between stator elements thereof,said capacitance providing electrostatic coupling between said circuits,and a single means for simultaneously adjusting said tuning condensers.

11. A high frequency network comprising a pair of resonant oscillationcircuits having xed inductors, means for shielding said inductorswhereby there exists substantially zero mutual inductance between them,variable tuning condensers connected to said inductors having a Amutualcapacitance between stator elements thereof, said capacitance providingelectrostatic coupling between said circuits, and a singlel means forsimultaneously adjusting said tuning condensers.

12. In combination, in a high frequency transmission system, a source ofhigh frequency energy, a tunable oscillation circuit coupled thereto, ahigh frequency space discharge amplifier, a second tunable oscillationcircuit between said first circuit and said amplifier input circuit, andcompound coupling means between said first and second tunable circuitsconsisting of a capacitor common to said tunable circuits and a bridgingcapacitor between said tunable circuits, said capacitors being soproportioned as to provide a substantially constant width of pass bandwith variable tuning of said circuits.

13. In combination, in a high frequency trans- 50 mission system, asource of high frequency energy, a tunable oscillation circuit coupledthereto, a high frequency space discharge amplifier, a second tunableoscillation circuit'between said rst circuit and said amplier inputcircuit, com- Y pound coupling means between said first and secondtunable circuits consisting of a capacitor common to said tunablecircuits and a bridging capacitor between said tunable.` circuits, saidcapacitors being so proportioned as to provide a substantially constantwidth of pass band with variable tuning of said circuits, and means forsimultaneously and similarly tuning said oscillation circuits to adesired frequency.

14. A receiver comprising a pair of adjacent radio frequency amplifyingstagesfeach stagebeing provided with'aV tunedcircuitincluding'capacitive tuning means, said capacitive tuning meansof both stages being sol constructed and arranged'as toprovidecapacitiveV coupling between the respective tuned circuits of saidadjacent stages, and a capacitorl common to said tuned circuits, saidcapacitive couplingY and said common capacitor being so proportionedasto'provide a substantially constant Width of pass band withy syntonoustuning of the circuits.

15. In a receiver, a stage of radio frequency amplification, asubsequent radio stage., a. tunable circuit Yassociated with` each ofAsaid stages, tuning means in each of said circuits for tuning the latterto any desired` frequency ini a-predetermined range of frequencies, saidtuning means'comprising a. pairof adjacent variable condensers eachhaving; a plurality of stator and rotor plates, the rotor' plates beingmounted on a common control shaft, coupling between the said stagesbeing provided by the, capacity between adjacent stator plates of Y theadjacent condensers, and additional coupling between the saidstages/being provided by' aV capacitor common vto both of said tunable10- circuits,` whereby a substantially constant band width is maintainedwith syntonous tuning of Ythe circuits. Y i

JOHNS. Low. i

